The present invention relates to electrical circuit elements made of carbon-containing glasses and is particularly concerned with electrical contact systems for such circuit elements which resist deterioration during exposure to high temperatures.
Glasses comprising an electrically conductive carbon phase are known, being first described by R. B. Ellis in U.S. Pat. No. 2,556,616. In general, such glasses are prepared by impregnating a porous glass body with one or more ingredients which can subsequently be decomposed to carbon. The porous glass body is typically a porous 96% silica glass body provided in accordance with the teachings of U.S. Pat. No. 2,106,744 to Hood et al.
The porous glass is impregnated by immersion into a solution or mixture comprising a suitable organic compound and is then heated to a temperature sufficient to decompose compound remaining in the pore structure to carbon. Thereafter, a high-temperature heating step under non-oxidizing conditions is used to consolidate the porous glass (collapse the pores), sealing in the conductive carbon phase and protecting it from subsequent oxidation. The electrical properties of the resulting product, which consists of a glass matrix surrounding a continuous, interconnected, thread-like carbon phase of comparatively low volume, are quite stable up to the pore consolidation temperature used in production.
Following the work of Ellis, a number of alternate methods for providing carbon-containing glasses from porous glasses, and particularly porous 96% silica glasses, were developed. U.S. Pat. No. 3,813,232 to Forker et al. and U.S. Pat. No. 3,775,078 to Elmer et al., among others, describe some of the alternate methods which have been employed.
Prior art carbon impregnation methods are generally suitable for introducing carbon into continuously porous ceramics or glasses regardless of the particular composition thereof. However, porous glasses of the type known in the art as 96% silica glasses are preferred from the standpoint of processing convenience, and provide desirable properties in the product. As in the prior art, the designation 96% silica glass is used herein in the generic sense to refer to all highly siliceous glasses produced by the method of the aforementioned Hood et al. patent, regardless of the exact silica content of the glass.
Carbon-containing glasses made from 96% silica glasses are low in thermal expansion and quite refractory, and having stable electrical properties can therefore function as electrical circuit elements at very high temperatures. However, difficulties in providing long term electrical contact with the carbon phase in the glass, even at moderate temperatures, are encountered.
Known methods for providing electrical contact with carbon-containing glasses usually involve the application of conductive graphite or metal-containing pastes or solutions to the glass to make contact with the exposed carbon phase. Alternatively, electroplating methods can be used to coat the exposed carbon with copper or other metals.
Contacts provided by these and similar methods usually deteriorate rapidly during thermal cycling. A major factor accelerating deterioration is the thermal expansion mismatch between low-expansion carbon-containing 96% silica glass (less than about 0.00009 percent per degree centigrade) and most metals useful for contacts (in the range of about 0.0007-0.0030 percent per degree centigrade). High expansion metals tend to break away from the largely glass contact surface area during temperature cycling, reducing the area of electrical contact and increasing current density. The resulting heating causes further thermal stress, and deterioration accelerates until destructive breakdown is complete.
It is the principal object of the present invention to provide an electrical contact for carbon-containing glass which resists deterioration even at high use temperatures and during repeated thermal cycling.
It is a further object of the invention to provide electrical circuit elements consisting of carbon-containing glass which include one or more electrical contacts suitable for use at high temperatures.
It is a further object of the invention to provide electrical heating elements which include high-use-temperature contacts which withstand extended thermal cycling.
Other objects and advantages of the invention will become apparent from the following description thereof.